The present invention relates to an automatic adjuster for the operation gap of a brake.
An automatic adjuster built in the wheel cylinder of a drum brake as disclosed in the U.S. Pat. No. 4,530,424, for examples, is known as a conventional automatic adjuster for the operation gap of a brake. Since the automatic adjuster is constituted so that at the time of the elastic deformation of a brake drum, the rise in liquid pressure over a prescribed level is utilized to prevent excessive adjustment, the constitution of the automatic adjuster is likely to be complicated and the life of a cup seal is relatively short. Besides, since the automatic adjuster is installed in a liquid pressure chamber, the efficiency of work for the installation is low.
Another conventional automatic adjuster is installed at the outer end of a wheel cylinder as disclosed in the Japanese Pat. No. 12974/72, for example. The automatic adjuster is provided with a mechanism for preventing excessive adjustment caused by the elastic deformation of a brake drum. In the automatic adjuster, an elastic member is inserted between a first and a second pistons extending in the axial direction of the adjuster and disposed coaxially with each other. The elastic member needs to have such elastic characteristics that it is scarcely deformed by the resilient force of a shoe return spring; and after a brake friction member comes into contact with the brake drum, the elastic member is compressed in the axial direction thereof but expanded in the radial direction thereof to couple a bushing and the operating piston to each other. However, the elastic member is likely to deteriorate with the lapse of time, it is difficult to maintain the elastic characteristics for a long period of time, and the life of the elastic member is relatively short.